How long do IO's stamp your passport for? the time you ask for? OR the 6 months

Since I am travelling outside america for the first time as a visitor, I have lots of questions and i am nervous.

I am planning to take a month vacation to visit a guy i have been talking to online. We have not met before and I am looking forward to meeting him.  I want to book my flight for 1 month.  (i will do an open ended ticket so i can change my return date if anything goes wrong.)

But i have always wanted to visit England and i would love a month to travel around with him and he can show me all the spots i want to go see and not have to cram it all into a few weeks so that is why i want to take a month out.

So I will book a flight/return for 1 month and I will inform the IO when they ask how long i am intending to stay.  I have my return ticket right there as proof.

Now, if they decide to let me in, do they actually give me only a 1 month's worth of time passport stamp and no longer? (since that is what I have booked the flight for)

-OR-

Do I automatically get a clearance of 'up to' six months max stay stamped on my passport?

Generally you get clearance for six months.

Actually it depends on how credible you are when you present yourself to the Immigration Officer. They can refuse you entry, allow you in for a couple of days, allow you in for the time that you request or allow you in for an entire 6 months.

However, one thing you need to remember is that if you tell the immigration officer that you're staying a month, then stay that month and leave when you said you were going to. UKBA can look in your travel history and see if you left when you said you were going to. If you stay longer than you said you were going to stay it could be perceived as you used deception (lied to the immigration officer) to gain entry by saying you were only going to stay a month and you stayed longer.

I would hazard a guess that the majority of US tourists are stamped in for six months, which is why I said 'generally'.

I've been to the UK 4 times in the past 5 months (5th visit tomorrow!) each stamp I received was the 6 month visa stamp. Every time I went was only a short visit, max one week and never overstayed the original time I told immigrations.

Always be honest and leave when you intend to...like WebyJ said...

I have a stamp from a past visit that put my exit date as the date I had booked to fly out.  When they gave it to me I didn't argue, I was intending to leave and did.  I did, however, cross my fingers and hoped that something like, a vocano didn't happen.  I've gotten the cleared for 6 months stamp since then. 

I'm not sure how they track exits though and would love more detail.  Whenever I leave, they just look at my passport for my old stamp and then send me on my way.  Nothing seems to get scanned.  I guess if I had left late, that would have been noted.  But then what does happen if you have that date out stamp and for whatever reason all air traffic is stopped? 

But then what does happen if you have that date out stamp and for whatever reason all air traffic is stopped? 

On the two times that air travel has been stopped from the UK to the US (9/11 and the Icelandic volcano) this is what UKBA did:

Notice for holders of expired visas whose travel plans have been disrupted by the volcanic ash

17 April 2010

The UK Border Agency would like to reassure travellers who have been unable to leave the UK and whose visas have now expired. We appreciate that this is due to exceptional circumstances beyond your control.

Please ensure that you retain proof of planned travel that would have enabled you to leave the UK before your visa expired. Please only contact the agency for further advice in exceptional circumstances. We will update this advice as the situation develops.

Just realise that there is a difference from intentionally lying to the IO by saying that you're going to stay 3 weeks in order to be stamped in and then changing your flight to stay longer, as opposed telling the IO that you want to stay 6 months, being stamped in for 6 months and then something happening that genuinely delays your travel such as illness, terrorrism or natural disaster.

Thanks!